1. Field of the Invention
This invention is concerned with analysis of organic additives and contaminants in plating baths as a means of providing control over the deposit properties.
2. Description of the Related Art
Electroplating baths typically contain organic additives whose concentrations must be closely controlled in the low parts per million range in order to attain the desired deposit properties and morphology. One of the key functions of such additives is to level the deposit by suppressing the electrodeposition rate at protruding areas in the substrate surface and/or by accelerating the electrodeposition rate in recessed areas. Accelerated deposition may result from mass-transport-limited depletion of a suppressor additive species that is rapidly consumed in the electrodeposition process, or from accumulation of an accelerating species that is consumed with low efficiency. The most sensitive methods available for detecting leveling additives in plating baths involve electrochemical measurement of the metal electrodeposition rate under controlled hydrodynamic conditions for which the additive concentration in the vicinity of the electrode surface is well-defined.
Cyclic voltammetric stripping (CVS) analysis [D. Tench and C. Ogden, J. Electrochem. Soc. 125, 194 (1978)] is the most widely used bath additive control method and involves cycling the potential of an inert electrode (e g., Pt) in the plating bath between fixed potential limits so that metal is alternately plated on and stripped from the electrode surface. Such potential cycling is designed to establish a steady state for the electrode surface so that reproducible results are obtained. Accumulation of organic films or other contaminants on the electrode surface can be avoided by periodically cycling the potential of the electrode in the plating solution without organic additives and, if necessary, polishing the electrode using a fine abrasive. Cyclic pulse voltammetric stripping (CPVS), also called cyclic step voltammetric stripping (CSVS), is a variation of the CVS method that employs discrete changes in potential during the analysis to condition the electrode so as to improve the measurement precision [D. Tench and J. White, J. Electrochem. Soc. 132, 831 (1985)]. A rotating disk electrode configuration is typically employed for both CVS and CPVS analysis to provide controlled hydrodynamic conditions.
For CVS and CPVS analyses, the metal deposition rate may be determined from the current or charge passed during metal electrodeposition but it is usually advantageous to measure the charge associated with anodic stripping of the metal from the electrode. A typical CVS/CPVS rate parameter is the stripping peak area (Ar) for a predetermined electrode rotation rate. The CVS method was first applied to control copper pyrophosphate baths (U.S. Pat. No. 4,132,605 to Tench and Ogden) but has since been adapted for control of a variety of other plating systems, including the acid copper sulfate baths that are widely used by the electronics industry [e.g., R. Haak, C. Ogden and D. Tench, Plating Surf. Fin. 68(4), 52(1981) and Plating Surf. Fin. 69(3), 62 (1982)].
Acid copper sulfate electroplating baths require a minimum of two types of organic additives to provide deposits with satisfactory properties and good leveling characteristics. The suppressor additive (also called the xe2x80x9cpolymerxe2x80x9d, xe2x80x9ccarrierxe2x80x9d, or xe2x80x9cwetterxe2x80x9d, depending on the bath supplier) is typically a polymeric organic species, e.g., high molecular weight polyethylene or polypropylene glycol, which adsorbs strongly on the copper cathode surface to form a film that sharply increases the overpotential for copper deposition. This prevents uncontrolled copper plating that would result in powdery or nodular deposits. An anti-suppressor additive (also called the xe2x80x9cbrightenerxe2x80x9d, xe2x80x9cacceleratorxe2x80x9d or simply the xe2x80x9cadditivexe2x80x9d, depending on the bath supplier) is required to counter the suppressive effect of the suppressor and provide the accelerated deposition within substrate recesses needed for leveling. Plating bath vendors typically provide additive solutions that may contain additives of more than one type, as well as other organic and inorganic addition agents. The suppressor additive may be comprised of more than one chemical species and generally involves a range of molecular weights.
Acid copper sulfate baths have functioned well for plating the relatively large surface pads, through-holes and vias found on printed wiring boards (PWB""s) and are currently being adapted for plating fine trenches and vias in dielectric material on semiconductor chips. The electronics industry is transitioning from aluminum to copper as the basic metallization for semiconductor integrated circuits (IC""s) in order to increase device switching speed and enhance electromigration resistance. The leading technology for fabricating copper IC chips is the xe2x80x9cDamascenexe2x80x9d process (see, e.g., P. C. Andricacos, Electrochem. Soc. Interface, Spring 1999, p.32; U.S. Pat. No. 4,789,648 to Chow et al.; U.S. Pat. No. 5,209,817 to Ahmad et al.), which depends on copper electroplating to provide complete filling of the fine features involved. The organic additives in the bath must be closely controlled since they provide the copper deposition rate differential required for bottom-up filling.
As the feature size for the Damascene process has shrunk below 0.2 xcexcm, it has become necessary to utilize a third organic additive in the acid copper bath in order to avoid overplating the trenches and vias. Note that excess copper on Damascene plated wafers is typically removed by chemical mechanical polishing (CMP) but the copper layer must be uniform for the CMP process to be effective. The third additive is called the xe2x80x9clevelerxe2x80x9d (or xe2x80x9cboosterxe2x80x9d, depending on the bath supplier) and is typically an organic compound containing nitrogen or oxygen that also tends to decrease the copper plating rate. In order to attain good bottom-up filling and avoid overplating of ultra-fine chip features, the concentrations of all three additives must be accurately analyzed and controlled.
The suppressor, anti-suppressor and leveler concentrations in acid copper sulfate baths can all be determined by CVS analysis methods based on the effects that these additives exert on the copper electrodeposition rate. At the additive concentrations typically employed, the effect of the suppressor in reducing the copper deposition rate is usually much stronger than that of the leveler so that the concentration of the suppressor can be determined by the usual CVS response curve or dilution titration analysis [W. O. Freitag, C. Ogden, D. Tench and J. White, Plating Surf. Fin. 70(10), 55 (1983)]. Likewise, the anti-suppressor concentration can be determined by the linear approximation technique (LAT) or modified linear approximation technique (MLAT) described by R. Gluzman [Proc. 70th Am. Electroplaters Soc. Tech. Conf., Sur/Fin, Indianapolis, Ind. (June 1983)]. A method for measuring the leveler concentration in the presence of interference from both the suppressor and anti-suppressor is described in U.S. patent application Ser. No. 09/968,202 to Chalyt et al. (filed Oct. 1, 2001).
Analysis for the suppressor additive typically involves generation of a calibration curve by measuring the CVS rate parameter Ar in a supporting electrolyte, termed Ar(0), and after each standard addition of the suppressor additive. The supporting electrolyte typically has the same organic content as the plating bath being analyzed but does not contain organic additives. The Ar parameter may be plotted against the suppressor concentration directly, or be normalized as Ar/Ar(0) to minimize measurement errors associated with changes in the electrode surface, background bath composition, and temperature. For the suppressor analysis itself, Ar is first measured in the supporting electrolyte and then after each standard addition of a known volume fraction of the plating bath sample to be analyzed. The suppressor concentration may be determined from the Ar or Ar/Ar(0) value for the measurement solution (supporting electrolyte plus a known volume fraction of plating bath sample) by interpolation with respect to the appropriate calibration curve (xe2x80x9cresponse curve analysisxe2x80x9d). Alternatively, the suppressor concentration may be determined by the xe2x80x9cdilution titrationxe2x80x9d method from the volume fraction of plating bath sample (added to the supporting electrolyte) required to decrease Ar or Ar/Ar(0) to a predetermined value, which may be a specific numerical value or a minimum value corresponding to substantially maximum suppression [W. O. Freitag, C. Ogden, D. Tench and J. White, Plating Surf. Fin. 70(10), 55 (1983)]. Note that the effects of the anti-suppressor and leveler additives on the suppressor analysis are typically small but can be taken into account by utilizing a background electrolyte (instead of the supporting electrolyte) containing the concentrations of these additives measured or estimated to be present in the plating bath being analyzed.
Suppressor additives in acid copper baths are typically supplied as long-chain polymers (e.g., polyethylene glycol or polypropylene glycol) of high molecular weight ( greater than 5000) but the polymer chains are cleaved during copper electrodeposition so that chemically similar species of lower molecular weight are produced. These lower-molecular-weight suppressor breakdown products are less effective at suppressing copper electrodeposition, and those with sufficiently low molecular weight represent contaminants that interfere with the electrodeposition process. To obtain acceptable electrodeposits, the concentrations of such suppressor breakdown contaminants must be maintained at low levels in the plating bath. This is normally accomplished by a bleed and feed operation in which a portion of the plating bath is continuously or periodically removed and replaced with fresh plating solution. A method is needed for monitoring the buildup of suppressor breakdown products so that the bleed and feed rates can be optimized to ensure acceptable electrodeposits, while minimizing consumption of expensive plating solution and generation of environmentally objectionable waste.
Available methods for detecting suppressor breakdown contaminants in acid copper baths are inadequate. The normal CVS standard addition or dilution titration analysis involving electrode potential cycling between fixed positive and negative limits yields the effective suppressor concentration but does not distinguish between high and low molecular weight species. Likewise, total organic carbon analysis does not provide information about the individual species present in the plating bath. The overall organic contaminant level in acid copper baths can also be detected via the diffusion-limited oxidation current at a platinum rotating disk electrode. This approach is widely used (in conjunction with CVS analysis) to detect contaminants in printed wiring board (PWB) plating baths but is not sensitive enough to detect the low levels of contaminants that interfere with plating of trenches and vias in the Damascene wafer plating process.
The present invention provides a sensitive method for determining the relative concentrations of active suppressor additive species and suppressor breakdown contaminants in an acid copper electroplating bath. In this method, the volume fraction of the plating bath added to the bath supporting electrolyte (or a background electrolyte) required to produce a predetermined decrease in the copper electrodeposition rate is determined for two predetermined copper deposition potentials or potential ranges. The volume fraction required for the more negative potential or potential range provides a measure of the concentration of the active suppressor additive since the suppressor breakdown contaminants are not effective at suppressing the copper deposition rate at the more negative potentials. The volume fraction required for the less negative potential or potential range provides a measure of the combined concentrations of the active suppressor additive and the suppressor breakdown contaminants A comparison (mathematical difference or ratio, for example) of the ,measured volume fractions for the two potentials or potential ranges yields the concentration of the suppressor breakdown contaminants relative to the active suppressor additive concentration. Both of these concentrations are averaged (or effective) concentrations since the active suppressor additive and the breakdown contaminants are each comprised of species having a range of molecular weights. The molecular weight ranges for the species detected depend on the specific electrode potentials used for the analysis.
In a preferred embodiment, CVS (or CPVS) dilution titration analyses are performed on an copper plating bath using two different negative potential limits. For these analyses, the CVS rate parameter (Ar) is first measured in the supporting electrolyte, termed Ar(0), and then after each standard addition (to the supporting electrolyte) of a known, volume fraction of the plating bath solution being analyzed. The volume fraction of plating bath solution corresponding to a predetermined value of Ar or Ar/Ar(0) provides a measure of the active suppressor concentration for the more negative potential limit, and the suppressor breakdown contaminant concentration plus the active suppressor concentration for the less negative potential limit. The predetermined value of Ar or Ar/Ar(0) may be a specific numerical value or a minimum value corresponding to substantially maximum suppression of the copper deposition rate. A calculated parameter based on a mathematical relationship between the solution volume fractions for the two potential limits (e.g., the ratio or difference) is used as a relative measure of the suppressor breakdown contaminant concentration. Since suppressor polymer chains of various lengths are produced during the copper electrodeposition process, calibration curves of the calculated parameter versus suppressor breakdown contaminant concentration are of limited use. On the other hand, the electrode potential limit used for the suppressor breakdown contaminant analysis may be adjusted to vary the molecular weight range of the species detected.
The analysis of the present invention is readily performed by repeating the normal CVS or CPVS suppressor additive analysis using a less negative cathodic potential limit (to determine the relative concentration of suppressor breakdown contaminants). For the CVS dilution titration approach, known volumes of the plating bath being analyzed are added to a known volume of the supporting electrolyte (or a background electrolyte) until Ar or Ar/Ar(0) reaches a minimum value corresponding to maximum suppression of the copper deposition rate, or Ar is decreased to a predetermined percentage (or fraction) of the Ar(0) value. The endpoint for the titration is typically the point at which maximum suppression is attained, or the Ar value reaches 50% of the Ar(0) value. For the response curve approach, the relative suppressor and suppressor breakdown contaminant concentrations are determined from the Ar or Ar/Ar(0) values measured at the two negative potential limits for a measurement solution (supporting electrolyte plus a known volume fraction of plating bath sample) by interpolation with respect to a suppressor calibration curve.
The present invention enables suppressor additive breakdown contaminants in acid copper baths to be analyzed and controlled so as to ensure acceptable copper electrodeposits while minimizing consumption of expensive plating solution and generation of environmentally objectionable waste. Such optimized plating bath performance may be attained via appropriate adjustments in the additive bleed and feed rates, and/or as-needed replacement of all or part of the plating bath. The present invention may also be used to determine the need for carbon treatment, which may be used to remove bath contaminants in some cases. In addition, the present invention may be applied to baths, employing polymeric additives, used to electrodeposits other metals, tin, tin-lead, nickel or cobalt, for example.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.